Despite hundreds of years of speculation and well over a century of focused research, an integrated description and understanding of the sex ratio in humans remains elusive. Using techniques and approaches from demography, economics, evolutionary biology, and medicine we will 1) analyze data derived from blastocysts in order to make an unprecedented estimate of the sex ratio at a defined time just after conception, 2) analyze data from chorionic-villus sampling and from amniocentesis in order to make accurate estimates of the sex ratio at well-defined times later during pregnancy, 3) analyze all published data on the sex ratios of induced abortions as well as published data on sex- specific fetal mortality after twenty weeks of pregnancy in order to describe the trajectory of the human sex ratio from early pregnancy to the neonatal period. We will also develop theoretical models in order to understand the demographic and evolutionary dynamics and equilibria associated with sex ratio biases in human populations and in order to derive testable predictions to apply to our data. The potential usefulness of a fuller understanding of the human sex ratio is substantial. In particular, our analyses of the early trajectory of the post-conception sex ratio will provide insight into the degree to which the sex ratio at birth is determined by sex-chromosome segregation and by other influences such as hormones, which are more environmentally labile. This research project on the sex ratio in human populations is of importance to public health because it will help us understand the degree to which the sex ratio is influenced by the biology of reproduction as well as by the family and social environments. Such information is useful for greater understanding of the causes and consequences of recent demographic trends in the United States. We will bring together contributions from demography, evolutionary biology, medicine, and statistics in order to gain a comprehensive understanding of the changes in the sex ratio during human pregnancy.